1. Field of the Invention
This invention relates to a semiconductor device, and more particularly to a semiconductor device based on a heterojunction structure of nitride semiconductors.
2. Background Art
Circuits like switching power supplies and inverters are based on power semiconductor devices such as switching devices and diodes, and the power semiconductor devices are required to have high breakdown voltage and low on-resistance. Between the breakdown voltage and the on-resistance, there is a tradeoff depending on the device material. By using nitride semiconductors such as GaN and AlGaN or wide bandgap semiconductors such as silicon carbide (SiC) as a device material, the tradeoff depending on the material can be improved relative to silicon to achieve low on-resistance and high breakdown voltage.
Devices based on nitride semiconductors such as GaN and AlGaN have good material characteristics, and hence high-performance power semiconductor devices can be realized. In particular, a HEMT (high electron mobility transistor) having an AlGaN/GaN heterostructure can realize low on-resistance because a high-concentration two-dimensional electron gas is generated by polarization at the interface between the AlGaN layer and the GaN layer.
However, in the HEMT, the generation of a high-concentration two-dimensional electron gas without impurity doping contrarily makes it difficult to realize a normally-off transistor. To achieve normally-off characteristics while maintaining low on-resistance, the two-dimensional electron gas concentration below the gate electrode needs to be selectively decreased.
To this end, for example, a recess gate structure is known (e.g., JP-A-2006-032650 (Kokai)), in which the AlGaN layer immediately below the gate electrode is etched to decrease the two-dimensional electron gas concentration only in the portion immediately below the gate electrode.
This recess etching is performed by dry etching such as reactive ion etching because AlGaN and GaN are chemically stable materials and difficult to wet etch. Here, the etching depth needs high accuracy in control and uniformity because the etching depth dictates the two-dimensional electron gas concentration. Furthermore, damage caused by dry etching is not negligible, which also causes concern about problems such as increased leakage current.
In a rectifying device (diode) based on nitride semiconductors, low on-resistance is compatible with high breakdown voltage. However, it is known that its leakage current at reverse bias is larger by several orders of magnitude than theoretically expected, which is a major problem for practical application.